1. Field of the Invention
The present invention relates to an image reading unit having photoelectric conversion elements and an image reading apparatus having the image reading unit.
2. Description of the Related Art
Conventionally, as an original illuminating method used to read a transparent original such as a photographic film by a flatbed type image reading apparatus having an original platen glass, a method has been mainly used in which a transparent original guide unit that holds a light-transmitting transparent original is placed on an upper surface of the original platen glass and the original is illuminated at an upper side thereof using a surface light source unit (refer to Japanese Laid-Open Patent Publication (Kokai) No. 2003-215733 and Japanese Laid-Open Patent Publication (Kokai) No. 2003-037713, for example). An image reading apparatus using this original illuminating method is generally designed such that the best focal position is at 0 mm on the upper surface of the original platen glass, that is, a position wherein the original directly contacts the original platen glass. This is because it is assumed that main objects to be read by the conventional image reading apparatus are reflective originals that reflect light.
When reading a transparent original by the image reading apparatus, the transparent original is placed up at a level of 0.5 mm above the original platen glass by the transparent original guide unit. The mounting of a transparent original at such a level above the original platen glass is employed, for example, prevention of a Newton rings (interference pattern), and prevention of the image reading unit from reading any contamination of the transparent original when the transparent original is placed in contact with the original platen glass.
The above method of illuminating a transparent original at an upper side thereof by the surface light source unit presents no problems in practical-use even if the distance between the original platen glass and the transparent original is on the order of 0.5 mm, when this method is used in combination with a lens of a magnification optical system adopting a CCD used as a lens mounted in the image reading unit, since the magnification optical system lens has a large depth of field.
In recent years, with a view to miniaturization of an image reading unit, a unity magnification image reading unit using a rod lens array has been proposed.
However, the rod lens array of this unity magnification image reading unit has a shallow depth of field, and therefore, as is distinct from the above magnification optical system lens using a CCD, when the distance between the original platen glass and the transparent original is on the order of 0.5 mm, there arises a problem that a blurred read image which is out of focus is obtained.
Moreover, the conventional image reading apparatus is mainly intended to read reflective originals as objects to be read, and are, therefore, not able to accurately read both reflective originals and transparent originals.